An alternating current (AC) drive system refers to a new drive system taking a motor as a control subject in which the speed and the torque of an AC motor is adjusted in a Variable Voltage Variable Frequency (VVVF) mode. The AC drive system is generally composed of a main circuit, a control system and a control subject, i.e., the AC motor. The main circuit includes a direct current (DC) bus, a DC support capacitor and a converter composed of a power semiconductor device. The control system is built on a hardware platform, e.g., a Digital Signal Processor (DSP), a Central Processing Unit (CPU), etc., and with use of a real-time control system on various AC motor control theories of, e.g., sliding difference, field oriented control, direct torque control, etc., it can acquire and process signals, e.g., the speed of the motor, the current of the motor, the voltage of the DC bus, etc., in the drive system, and control the power semiconductor device in the main circuit to be on and off in response to a required speed or torque instruction to adjust the amplitude and the frequency of an AC voltage acting on the motor, thereby controlling the speed and the torque of the motor.
Pulse Width Modulation (PWM) is one of extremely important components in the AC drive control system, which functions to adjust the width of a pulse signal controlling the power semiconductor device of the main circuit to be on and off in response to an input reference voltage and an ongoing voltage of the DC bus to make a fundamental wave voltage output from the main circuit equal to the input reference voltage. PWM can be categorized into asynchronous modulation and synchronous modulation by different modulation ratios, in the former of which the switching frequency of the converter keeps unchanged. With synchronous modulation, the switching frequency of the converter strictly keeps a proportional relationship with the fundamental wave frequency output from the converter so that the switching frequency varies with the fundamental wave frequency. A significant advantage of synchronous modulation over asynchronous modulation lies in not only keeping symmetry of a three-phase AC output from the converter but also attaining Half Wave Symmetry and Quarter Wave Symmetry of a phase voltage to thereby reduce the number of low-order harmonic waves. Synchronous modulation is commonly used in a high speed zone of a high power drive system.
General methods of triangular carrier wave comparison and polygonal trace tracking are currently available to synchronous modulation, the former of which will firstly be introduced below.
In the triangular carrier wave comparison method, a three-phrase modulation wave of the converter is compared with the same triangular carrier wave to output a three-phrase PWM signal, and the ratio of the frequency of the triangular carrier wave to that of the modulation wave keeps unchanged to ensure a strict proportional relationship between the switching frequency of the converter and the fundamental wave frequency output from the converter. In order to address such a disadvantage that the switching frequency is so low in the case of a low frequency that the number of harmonic waves may be increased and so high in the case of a high frequency that it may be difficult for the device to be tolerant, segmented synchronous modulation can be adopted so that a ranges of frequency output from the converter is divided into several frequency bands with a carrier wave ratio which keeps constant in each of the frequency bands but varies from one frequency band to another. Reference is made to FIG. 1 illustrating a schematic diagram of segmented synchronous modulation in the prior art. The slope of a solid line in FIG. 1 represents a carrier waver ratio increasing segment-by-segment as the increasing frequency of the modulation wave, and a dotted line above represents an upper limit of the switching frequency of the converter. 0˜f1 or f2˜f3 represents a frequency band. Reference is made to Table 1 in which carrier wave ratios of respective frequency bands are listed.
TABLE 1Carrier wave ratios of respective frequency bandsFrequencyCarrier wave ratio0~f1N1f1~f2N2f2~f3N3. . .. . .
The triangular carrier wave comparison method includes the following steps:
Step 101:The frequency f of the modulation wave is sampled.
Step 102: A carrier wave ratio N corresponding to the frequency in the step 1 is retrieved from Table 1 by using the frequency.
Step 103: A corresponding angle Δθ=2π/N is determined from the carrier wave ratio N.
Step 104: A timing value corresponding to the modulation wave is derived from the angle Δθ as T=Δθ/ω=Δθ/2πf=1/Nf and transmitted to a first timer.
Step 105: A modulation ratio m is retrieved from a modulation ratio vs. frequency graph in the prior art illustrated in FIG. 2A. The modulation ratio is defined as m=Vs/Vdc, where Vdc represents a voltage at the DC side, and Vs represents the amplitude of a reference voltage vector.
Step 106: The first sine values of U, V and W are retrieved from a sine table.
Step 107: Periods of time during which U, V and W phase switches are on and off are calculated respectively in the formula
                    {                                                                              T                  1                                =                                                      T                    2                                    ⁢                                      (                                          1                      +                                              m                        ⁢                                                                                                  ⁢                        sin                        ⁢                                                                                                  ⁢                        2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  ft                          d                                                                                      )                                                                                                                                                                T                    1                    ′                                    =                                      T                    -                                          T                      1                                                                      ,                                                                        (        1        )            and the off periods of time are transported to second, third and fourth timers, where m represents the modulation ratio, T represents a control cycle, T1 represents a period of time during which a switch is on, T1 represents a period of time during which a switch is off, f represents the frequency of the modulation wave.
Step 108: An interruption is enabled, a changed-frequency flag is read, and if the frequency has been changed, the flow jumps to the step 102; otherwise, the flow continues with making determination.
In the step of interrupting the first timer, it is determined whether the number of samples reaches N, and if so, the frequency of the modulation wave is sampled, and the frequency is determined whether it has been changed, and if the frequency has been changed, the changed-frequency flag is set. Or if the number of samples does not reach N, subsequent sine values of U, V and W are retrieved from the sine table. Periods of time during which the U, V and W phase switches are on and off are calculated respectively in the formula (1), and the off periods of time are transported to the first, second and third timers.
In the step of interrupting the second, third and fourth timers, the interrupted timers are determined so that the first timer is for the U phase, the second timer is for the V phase and the third timer is for the W phase. It is determined whether the number of samples is odd or even so that a switch signal is output as one if it is odd or zero if it is even. Timing values are updated with the on periods of time.
In the polygonal trace tracking method, when the speed of the motor is not very low, the voltage drop across the resistance of a stator can be neglected, and the vector relationship between the stator voltage {right arrow over (V)}s and the stator magnetic linkage {right arrow over (ψ)}s of the asynchronous motor can be derived as
                                          V            ⇀                    s                =                                            ⅆ                              ⅆ                t                                      ⁢                          (                                                ψ                  s                                ⁢                                  ⅇ                                      jω                    ⁢                                                                                  ⁢                    t                                                              )                                =                                    ωψ              s                        ⁢                                          ⅇ                                  j                  ⁡                                      (                                                                  ω                        ⁢                                                                                                  ⁢                        t                                            +                                              π                        /                        2                                                              )                                                              .                                                          (        2        )            As can be apparent in the formula (2), {right arrow over (V)}s is proportional to the angular frequency and directionally orthogonal to the stator magnetic linkage {right arrow over (ψ)}s when the amplitude of the {right arrow over (ψ)}s is constant. As a magnetic linkage vector is rotated by 360 degrees in the space, the voltage vector is also moved continuously in the tangential direction of the magnetic linkage circle by 2π in a trace colliding with the magnetic linkage circle. Thus, the issue of a trace along which the magnetic linkage of the AC motor is rotated can be translated into the issue of a trace along which a voltage space vector is moved. Ideally, it is desirable for the trace of the magnetic linkage to be a circle, but a voltage space vector is limited for the converter of a two-level voltage type, which makes it impossible for the magnetic linkage to be a circle, and consequently a circle has to be replaced with the most approximate polygonal to a circle. Variable polygonal trace tracking will be described below taking a normal dodecagon as an example. Reference is made to FIG. 3 illustrating a normal dodecagon in the polygonal trace tracking method in the prior art. A circle is replaced with the normal dodecagon in which six edges can be generated directly from a non-zero voltage vector and the other six edges have to be generated from synthesis of vectors to result in a polygon with thirty edges. Reference is made to FIG. 4 illustrating a thirty-edge polygonal trace of a magnetic linkage in the polygonal trace tracking method in the prior art. Along with the increasing frequency of the modulation wave, the carrier wave ratio is decremented, and the polygon with thirty edges is converted into a polygon with eighteen edges. Reference is made to FIG. 5 illustrating an eighteen-edge polygonal trace of a magnetic linkage in the polygonal trace tracking method in the prior art. Finally the polygon is converted into a hexagon to thereby result in a square wave. Specific steps 201 to 205 thereof are identical to the steps 101 to 105 in the triangular carrier wave comparison method and repeated descriptions thereof will be omitted here, and only those subsequent different steps will be introduced.
Step 206: T1, T2 and T0 are calculated in the formula
                                       {                                                                                          T                    1                                    =                                                            3                                        ⁢                    mT                    ⁢                                                                                  ⁢                                          sin                      ⁡                                              (                                                                              π                            3                                                    -                                                      θ                            m                                                                          )                                                                                                                                                                                      T                    2                                    =                                                            3                                        ⁢                    mT                    ⁢                                                                                  ⁢                                          sin                      ⁡                                              (                                                  θ                          m                                                )                                                                                                                                                                                      T                    0                                    =                                      T                    -                                          T                      1                                        -                                                                  T                        2                                            .                                                                                                                              (        3        )            
Step 207: A zero vector is segmented, and acting periods of time of respective minor steps of the vector are determined and transported to a buffer area.
Step 208: The timing value T is transported to the first timer and an interruption is enabled.
Step 209: If the value in the buffer area has been retrieved, the flow goes to the next step; otherwise, the flows waits.
Step 210: If the number of times that calculation has been performed is below N/6, the flow jumps to the step 207; otherwise, the flow goes to the step 201.
In the step of interrupting the first timer, the data in the buffer area is retrieved, and the voltage vector of the first segment is output, and the timing corresponding to the voltage vector of the first segment is transported to the second timer.
In the step of interrupting the second timer, the voltage vector of the next segment is output, and the timing corresponding to the voltage vector of the next segment is transported to the second timer.
In both of the foregoing methods of triangular carrier wave comparison and polygonal trace tracking, calculation is performed with a temporal reference so that firstly the carrier wave ratio N, i.e., the number of samples, is determined from the frequency f, and then the angle Δθ that the samples have undergone is determined from the frequency f and the number of samples N, the period of time T that the samples have undergone is calculated from the undergone angle Δθ, respective PWM output periods of time are calculated in the formula (3) and transported to the timers, and respective voltage vectors are output during the respective periods of time for the purpose of outputting respective angles. Both of the methods have to convert an angle into a period of time for calculation and then provide a PWM output by means of a timer, which may result in a complex calculation process, and moreover a timing value is determined from the frequency of the modulation wave, but an input frequency may vary in the meantime, which may result in consistency of an actual PWM output angle with a predetermined angle, thus degrading the performance of and even frustrating the purpose of synchronous modulation.